Many types of devices for proper functioning depend on specialized characteristics of the materials from which the devices are made. Steel must be strong enough and hard enough to serve as a good drill bit material. A bearing material must be made of a material that is adequately hard and yet machinable to adequate smoothness. Electrical wire must have adequate conductive and ductility characteristics. The list is endless.
One particular set of characteristics that is often important in testing materials is the submicroscopic, or nanometer structure. In particular, designs of various types of devices depend on the submicroscopic surface structure of materials from which the devices are made. One example is in computer hard drives, where these characteristics of both the individual disks and the heads that read and write the data affect the allowable bit density and the bearing characteristics of the head and disk surfaces.
A number of devices now have the ability to measure various mechanical characteristics of a material surface on the nanometer level. For example, an atomic force microscope (AFM) or a scanning probe microscope (SPM) can perform scratch, indent, and wear tests using a tool such as a small stylus having a hard, typically diamond, tip. SPMs and AFMs typically use an actuator capable of precise positioning of a tool mounted on any of them. Such actuators will be referred to generally hereafter as precision positioners.
Revealing the near-surface internal structure of many types of engineering materials on a submicroscopic level is often very useful. The internal structure of interest may be the crystal grain shapes, orientations, boundaries and distributions; concentration of contaminants; inclusions; voids; or other features. One example of such conventional devices is disclosed in U.S. Pat. No. 5,866,807.
This has been done in the past by a process that relies in part on precision positioner imaging. The first step in this process is to highly polish the surface. Then lightly etching the material surface with an appropriate etchant alters the surface topography in a way that reveals internal structure. This surface topography may be very small and detailed, and quite impossible to see with a conventional optical microscope. Then a precision positioner is used to obtain data values from which an image of the surface topography can be formed. This image provides an indication of the material's internal structure.
Conventionally, force and motion oriented in the plane of a workpiece surface can be defined by reference to orthogonal X and Y-axes parallel to the nominal surface of the workpiece. A Z-axis normal to the X and Y-axes thus defines force and motion vectors normal to the nominal surface of the workpiece.
A precision positioner commonly has a three axis piezoelectric element that can precisely move a mounted tool along the X Y, and Z-axes under control of a positioning signal, and accurately measure the coordinates of the tool at any time. By forcing the tool toward the workpiece surface along the Z-axis, the precision positioner can cause the tool to scribe or dent the surface. Force measurements during such machining activity or analysis of the machined surface thereafter allow one to accurately determine material characteristics such as hardness and wear resistance. The commonly available precision positioners have ample force available to perform the scribing or denting required by the invention.
Precision positioners can accept and manipulate various types of auxiliary attachments. One type of auxiliary attachment that can be mounted on a precision positioner is a type of force transducer called a nanoindenter. A nanoindenter allows a tool carried by the nanoindenter to be pressed against the workpiece surface with an exact amount of force.
A nanoindenter typically carries as its tool, a small diamond-tipped stylus that the nanoindenter can use to scribe or dent a workpiece surface. This invention most often uses a nanoindenter carrying a stylus or tool having a conical or pyramidal tip with an effective radius of curvature ranging from 30 to 1000 nanometers (nm). Nanoindenters can be calibrated to accurately provide Z-axis force ranging from 100 nanonewton (nN) to perhaps 30 millinewton (mN).
An enterprise named Hysitron located in Eden Prairie, Minn. makes a version of a nanoindenter preferred for this invention. The Hysitron nanoindenter has a structure explained in U.S. Pat. Nos. 5,553,486 and 5,576,483.
The Hysitron nanoindenter carries the tool on a special spring. The Hysitron nanoindenter spring allows the tool to translate along the Z axis, and very strongly resists translation and rotation in the other five degrees of freedom. When the precision positioner forces the tool into a workpiece surface, the spring is deflected.
The Hysitron nanoindenter can operate in two modes, precise Z-axis positioning and precise measurement of Z-axis force. The latter function is of particular interest for this invention. In force measurement mode, the Hysitron nanoindenter provides a tip force signal indicating the Z-axis force that the tool tip applies to the workpiece surface. The Hysitron nanoindenter measures force from the deflection of the internal spring. The amount of spring deflection precisely indicates the Z-axis force currently applied to the tool.
The Hysitron nanoindenter includes components that allow the spring deflection to be measured. The moving element of the Hysitron spring carries a plate whose spacing from adjacent control surfaces can be precisely determined using changes in capacitance between the plate and adjacent control surfaces to alter an AC signal applied between the plate and the control surfaces. The technical aspects of measuring spring deflection are explained in the previously identified U.S. Pats.
A Hysitron nanoindenter can provide and measure a wide range of tip forces, from a large force that causes the tool tip to penetrate relatively deeply into a particular workpiece, to a low force that penetrates into the workpiece very little, if any.
FIG. 1 shows a conventional nanoindenter test unit 1 of which a precision positioner 13 forms a part. Test unit 1 has a frame 10 having a central opening in which the various elements of test unit 1 are mounted. Precision positioner 13 is mounted in the central opening above a stage or table 22 on which a workpiece 11 is mounted. Frame 10 has extreme rigidity and a large mass to prevent vibration and movement of frame 10 that may affect the accuracy with which positioner 13 is supported above stage 22. Positioner 13 carries a transducer 18 attachment having a tool tip 21 adjacent to a surface on workpiece 11 to be tested.
Precision positioner 13 is shown separately from frame 10 in FIG. 2. Preferably, transducer 18 is a nanoindenter of the type available from Hysitron mentioned earlier. Tool 21 has a tip 21 a that typically is formed of diamond having, as previously mentioned, a radius of curvature ranging from 30 to 1000 nanometers (nm). The tip 21a usually has a conical or a three or four-sided pyramidal shape.
A preferred type of positioner 13 comprises a piezoscanner unit for precision X, Y, and Z-axis positioning of nanoindenter 18 and the attached tool 21. Such a positioner 13 can, under the control of a tool positioning signal carried on signal path 17a, shift tool 21 with a precision on the order of nanometers to any desired X, Y, and Z-coordinates within the range of movement of positioner 13. Positioner 13 provides a tip coordinate signal on path 17b that specifies with excellent accuracy the current X, Y, and Z coordinates of tip 21a relative to some origin.
Transducer 18 provides a tip force signal on path 16. The tip force signal precisely specifies the force applied along the Z-axis to the tip 21a by the workpiece 11 surface. As previously mentioned, this force can be inferred from the amount of spring deflection experienced by transducer 18 along the Z-axis since the relationship between force and deflection of the spring along the Z-axis can be determined with high accuracy.
Controller 20 further provides the positioning signal to positioner 13 on path 17a. The positioning signal directs positioner 13 to move tool tip 21a to a specified set of X, Y, and Z coordinates. Thus, by holding say the Y and Z coordinates constant and changing the X coordinate incrementally, controller 20 can cause precision positioner 13 to shift tool tip 21a along a line parallel to the X-axis.
A stage actuator 19 carries stage 22. Actuator 19 can move stage 22 through at least several cm in the X, Y, and Z-axes for gross positioning of workpiece 11, and then lock at a desired position. Often, controller 20 operates actuator 19 when positioning stage 22. Controller 20 uses positioner 13 for precisely positioning the tool tip 21a carried by the transducer 18 at the X and Y coordinates specified by the positioning signal.